[Python-Dev] PEP 455: TransformDict

[Ethan Furman]

On 10/07/2013 02:24 PM, Steven D'Aprano wrote:
> On Fri, Oct 04, 2013 at 11:06:15PM +0200, Victor Stinner wrote:
>
> if type(self) is not dict:
>      # This only applies to subclasses, not dict itself.
>      try:
>          transform = type(self).__transform__
>      except AttributeError:
>          pass
>      else:
>          key = transform(key)
> # now use the key as usual
>
>
> Am I barking up the wrong tree? Would this slow down dict access too
> much?

Considering that __transform__ would usually not exist, and triggered exceptions are costly, I think it would.

 From the docs[1]:

(10)
     If a subclass of dict defines a method __missing__, if the key k is not present, the a[k] operation calls that 
method with the key k as argument. The a[k] operation then returns or raises whatever is returned or raised by the 
__missing__(k) call if the key is not present. No other operations or methods invoke __missing__(). If __missing__ is 
not defined, KeyError is raised. __missing__ must be a method; it cannot be an instance variable. For an example, see 
collections.defaultdict. New in version 2.5.

So something more like:

     transform =  getattr(self, '__transform__', None)
     if transform is not None:
         key = transform(key)
     ...

A key difference (pun unavoidable ;) between __missing__ and __transform__ is that __missing__ is only called when a key 
is not found, while __transform__ needs to be called /every/ time a key is looked up:

   d[k]
   d.get(k)
   d.has_key(k)
   d.fromkeys(...)
   d.setdefault(...)
   k in d

--
~Ethan~